I am planning a new workbench with a leg vise and came across this chain assembly for the parallel guide in a couple pictures.

I cannot find any other information on it, but it looks like it pulls the guide out as the chop is pulled out. The problem is that I do not see how the chain keeps the chop parallel when the chop is being tightened. Note that the parallel guide does not have holes and so is not pinned to avoid racking when the clamp is tightened. Am I missing something?

Just for vocabulary enhancement - I assume the chop is the part that on any other vice would be called the movable jaw. I see what you mean about this particular leg vice. When the screw is tightened, the chain should lose tension, become slack and it would become like pushing a string. Perhaps the user is required to push the bottom using his toe or knee?
– Ast PaceMar 5 '16 at 23:12

2 Answers
2

This drawing from Shigshop shows the part that is not visible in your photos.

Under the slide, mounted in the leg, is a wheel/bearing that allows the lower part of the chop to move in without binding. The chain keeps the far end of the slide at a constant height which keeps it from binding.

Shigshop is also a source for buying plans for a roubo style work bench - if you are so inclined.

Are you referring to the wheel in the first photo of the post?
– MattMar 6 '16 at 0:05

OK, so if I'm seeing this correctly, the chain goes from end of parallel guide, thru one roller and up the leg, then over another roller and out through the front of the leg and to the chop under/paralleling the lead screw. Effectively, this means the guide must move in and out the same distance as the screw (more or less), keeping the chop parallel to the leg. The roller underneath is just to keep the chop's height constant so this works. Interesting solution. I'd be interested in a kit, at some point, if it really works as advertised. (I can see a few ways it might not, in real life.)
– keshlamMar 6 '16 at 1:03

@Matt Well, gee, shucks, I guess I am. Didn't notice it until you mentioned it, but I'm guessing that OP must have missed it too, or there would never have been need for the question.
– Ast PaceMar 6 '16 at 1:12

@keshlam I would think the tension of the chain would have to exactly 'goldilocks' tight
– Ashlar♦Mar 6 '16 at 2:14

I am having a hard time to try and explain this but the jist of it is that the whole chop and parallel bar move together simultaneously. The chain ensures that both move at a steady rate. When installed properly of course. Some people comment that it does not move as fluidly as a normal parallel bar would (some resistance when turning the wheel).

Tightening the wheel moves the parallel bar back into the bench, as it is attached to the chop, which pulls the chain. The chain can never slack as the distance rung never changes. That I think is the important part. After reading information on several sites I could not find straight information about the movement of the chain.

In the build that you got the picture from you will see that some roller wheels were installed to help guide the bar and prevent racking.

Proper attribution is important. I found this image from a lumberjocks post of a gentleman in Russia outfitting his bench with this vise apparatus. It actually is a kit that is sourced from the US of mostly US parts.

Thanks Matt. The part that had me confused is that when clamping something at the top, the bottom of the leg wants to rack more towards the bench leg due to the screw acting as a pivot point. From this picture it looks like the bottom rail, which is further from the pivot point wants to move further in, but its movement is restricted to the shorter pivot distance of the chain just below the pivot on the chop end. Still, intuitively I would think this puts more pressure on the parallel rail to chop connection and requires a pretty tight fit for the rail through the bench leg.
– Ashlar♦Mar 6 '16 at 2:11

Hmm, the kit price is similar to the "X" scissors leg brace option found in other kits.
– Ashlar♦Mar 6 '16 at 2:12

@Ashlar Oh. I'm sure you know I'm not endorsing..... Just showing one place it comes from.
– MattMar 6 '16 at 2:15

If you follow links, there's another attempt at a self-adjusting "pinless" leg vise at the bottom of lllars.com/the_workshop/roubo_workbench/index.php whiuch is based on a wedge mechanism. Dead simple, but I'm not entirely convinced it does work across the whole range. It does point out that absolute parallelism isn't required -- you don't get that with the pin designs either -- it wants to pull into something close to parallel as it's tightened down.
– keshlamMar 6 '16 at 2:51